Method for low temperature bonding and bonded structure

ABSTRACT

A method for bonding at low or room temperature includes steps of surface cleaning and activation by cleaning or etching. The method may also include removing by-products of interface polymerization to prevent a reverse polymerization reaction to allow room temperature chemical bonding of materials such as silicon, silicon nitride and SiO 2 . The surfaces to be bonded are polished to a high degree of smoothness and planarity. VSE may use reactive ion etching or wet etching to slightly etch the surfaces being bonded. The surface roughness and planarity are not degraded and may be enhanced by the VSE process. The etched surfaces may be rinsed in solutions such as ammonium hydroxide or ammonium fluoride to promote the formation of desired bonding species on the surfaces.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims the benefit ofpriority under 35 U.S.C. §120 from U.S. Ser. No. 14/754,111, filed Jun.29, 2015, which is a division of U.S. Ser. No. 14/197,070, filed Mar. 4,2014, which is a division of U.S. Ser. No. 13/341,273, filed Dec. 30,2011, which is a continuation of U.S. Ser. No. 12/954,740, filed Nov.26, 2010, now U.S. Pat. No. 8,153,505, which is a continuation of U.S.Ser. No. 12/720,368 filed Mar. 9, 2010, now U.S. Pat. No. 7,871,898,which is a continuation of U.S. Ser. No. 11/980,664 filed Oct. 31, 2007,now U.S. Pat. No. 7,807,549, which is a continuation of U.S. Ser. No.10/913,441 filed Aug. 9, 2004, now U.S. Pat. No. 7,387,944, which is acontinuation of U.S. Ser. No. 09/505,283, filed Feb. 16, 2000, now U.S.Pat. No. 6,902,987, the entire contents of each of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to bonding of materials at roomtemperature and, in particular, to bonding of processed semiconductormaterials, such as integrated circuit or device substrates, havingactivated surfaces to achieve high bonding strength adequate forsubsequent fabrication and/or a desired application.

2. Background of the Invention

Direct room temperature bonding generally produces weak van der Waals orhydrogen bonding. Annealing is typically required to convert the weakbond to a stronger chemical bond such as a covalent bond. Other waferbonding techniques including anodic and fusion typically require theapplication of voltage, pressure and/or annealing at elevatedtemperature to achieve a sufficient bond strength for subsequentfabrication and/or the desired application. The need to apply voltage,pressure or heat has significantly limited wafer bonding applicationsbecause these parameters can damage the materials being wafer bonded,give rise to internal stress and introduce undesirable changes in thedevices or materials being bonded. Achieving a strong bond at lowtemperatures is also critical for bonding of thermally mismatched orthermally sensitive wafers including processed device wafers.

Ultra high vacuum (UHV) bonding is one of the approaches to achieve alow or room temperature strong bond. However, the bonding wafers stillhave to be pre-annealed at high temperatures, for instance >600° C. forsilicon and 500° C. for GaAs, before cooling down to low or roomtemperature for bonding. Furthermore, the UHV approach does notgenerally work on commonly used materials, for example, in SiO₂. It isfurther also expensive and inefficient.

Adhesive layers can also be used to bond device wafers to a variety ofsubstrates and to transfer device layers at low temperatures. However,thermal and chemical instability, interface bubbles, stress and theinhomogeneous nature of adhesive layers prevent its wide application. Itis thus highly desirable to achieve a strong bond at room temperature bybonding wafers in ambient without any adhesive, external pressure orapplied electric field.

Low vacuum bonding has been explored as a more convenient alternative toUHV bonding but a bonding energy comparable to the bulk silicon fractureenergy using bonded bare silicon wafer pairs has only be achieved afterannealing at ˜150° C. For oxide covered silicon wafer pairs annealing at˜300° C. is required to obtain a high bond energy. It has not beenpossible to obtain high bonding energies in bonded material using lowvacuum bonding at room temperature.

A gas plasma treatment prior to bonding in ambient is known to enhancethe bonding energy of bonded silicon pairs at low or room temperature.See, for example, G. L. Sun, Q.-Y. Tong, et al., J. de Physique, 49(C4),79 (1988); G. G. Goetz, Proc. of 1st Symp. on Semicond. Wafer Bonding:Science, Technol. and Applications, The Electrochem. Soc., 92-7, 65(1992); S. Farrens et al., J. Electroch. Soc., 142,3950 (1995) andAmirffeiz et al, Abstracts of 5th Symp. on Semi. Wafer Bonding: Science,Tech. and Appl., The Electrochemical Society, 99-2, Abstract No. 963(1999). Although these treatments have increased the bond energyobtainable at low or room temperature, they have only been demonstratedwith planar silicon wafers or with silicon wafers using a plasma processthat results in oxide being grown on the wafers during the plasmaprocess. Moreover, these treatments have only been used to increase thebond energy by charging or damaging the surface. Furthermore, thesetreatments have not been used or shown to be applicable to depositeddielectrics or other materials.

Obtaining low or room temperature bonding with a method that is not onlyapplicable to planar silicon and grown oxide surfaces but further todeposited materials and non-planar surfaces with planarized depositedmaterials will allow generic materials, including processedsemiconductor wafers, to be bonded with minimal damage for manufacturingpurposes. Such a method based on etching and chemical bonding isdescribed herein.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for bondingmaterials at low or room temperature.

It is another object of the invention to bond materials by cleaning andactivating the bonding surfaces to promote chemical bond formation atabout room temperature.

It is a further object of the invention to provide a bonding method tobond any solid state material such as processed device or integratedcircuit wafers or thermally sensitive or mis-matched materials at orabout room temperature.

It is further object of the invention to provide a bonding method tobond processed device or integrated circuit wafers of different types ofdevices or different technologies, and transfer a layer of devices orcircuits at or about room temperature.

It is another object of the invention to enable a direct wafer bondingmethod that does not require annealing to achieve a required bondstrength.

It is a further object of the invention to provide a method wherebydiverse materials including those with non-planar surfaces and depositedmaterials can be planarized and bonded.

These and other objects are achieved by a method of bonding having stepsof forming first and second bonding surfaces, etching the first andsecond bonding surfaces, and

bonding together at room temperature the first and second bondingsurfaces after said etching step. The etching may include etching thefirst and second bonding surfaces such that respective surfaceroughnesses of the first and second bonding surfaces after said etchingare substantially the same as respective surface roughnesses before saidetching. The surface roughness may be in a range of 0.1 to 3.0 nm.

The bonding surfaces may be the surface of a deposited insulatingmaterial, such as silicon oxide, silicon nitride or a dielectricpolymer. The bonding surface may also be the surface of a silicon wafer.Silicon wafers, using either the surface of the wafer or a depositedmaterial on the wafer, may be bonded together. The wafers may havedevices or integrated circuits formed therein. The devices and circuitsin the wafers bonded together may be interconnected. The wafers may havea non-planar surface or an irregular surface topology upon which amaterial is deposited to form the bonding surfaces.

Forming at least one of the bonding surfaces may include depositing apolishable material on a non-planar surface. Depositing said polishablematerial may include depositing one of silicon oxide, silicon nitride ora dielectric polymer. The bonding surfaces may be polished using amethod such as chemical-mechanical polishing. The surfaces may also beetched prior to the polishing.

The etching step may also include activating the first and secondbonding surfaces and forming selected bonding groups on the first andsecond bonding surfaces. Bonding groups may also be formed capable offorming chemical bonds at approximately room temperature, and chemicalbonds may be formed between the bonding surfaces allowing bonded groupsto diffuse or dissociate away from an interface of the bonding surfaces.The chemical bonds can increase the bonding strength between the bondingsurfaces by diffusing or dissociating away said bonding groups.

After said etching step, the bonding surfaces may be immersed in asolution to form bonding surfaces terminated with desired species. Thespecies may comprise at least one of a silanol group, an NH₂ group, afluorine group and an HF group. Also, a monolayer of one of a desiredatom and a desired molecule may be formed on the bonding surface.Terminating the surface may include rinsing said bonding materials in anammonia-based solution after said slightly etching. The ammonia-basedsolution may be ammonium hydroxide or ammonium fluoride.

The method may also include exposing the bonding surfaces to one of anoxygen, argon, NH₃ and CF₄ RIE plasma process. Silicon dioxide may bedeposited as to form the bonding surfaces, and etched using the RIEprocess.

The etching process may create a defective or damaged zone proximate tothe bonding surfaces. The defective or damaged zone can facilitate theremoval of bonding by-products through diffusion or dissociation.

The method may also include steps of forming first and second bondingsurfaces, etching the bonding surfaces, terminating the bonding surfaceswith a species allowing formation of chemical bonds at about roomtemperature, and bonding the bonding surfaces at about room temperature,or may include steps of forming the bonding surfaces each having asurface roughness in a range of 0.1 to 3 nm, removing material from thebonding surfaces while maintaining said surface roughness, and directlybonding the bonding surfaces at room temperature with a bonding strengthof at least 500 mJ/m², at least 1000 mJ/m², or at least 2000 mJ/m².

The objects of the invention may also be achieved by a bonded devicehaving a first material having a first etched bonding surface, and asecond material having a second etched bonding surface directly bondedto the first bonding surface at room temperature having a bondingstrength of at least 500 to 2000 mJ/m². The bonding surfaces may bebeing activated and terminated with a desired bonding species, and thedesired species may include a monolayer of one of a desired atom and adesired molecule on said bonding surface or at least one of a silanolgroup, an NH₂ group, a fluorine group and an HF group. The bondingsurfaces may each have a defective region located proximate to saidfirst and second bonding surfaces, respectively.

The first material may include a surface of a first semiconductor waferhaving devices formed therein, and the second material may include asurface of a second semiconductor wafer having devices formed therein.Devices in the wafers may be interconnected, and the wafers may be ofdifferent technologies. The wafers may also have an integrated circuitformed therein, and devices or circuits in the wafers may beinterconnected.

One of said first and second wafers may be a device region afterremoving a substantial portion of a substrate of said one of said firstand second wafers. The wafers may have an irregular surface topology.

The first material may include a first wafer containing electricaldevices and having a first non-planar surface, and the first bondingsurface may include a polished and etched deposited oxide layer on saidfirst non-planar surface. The second material may include a second wafercontaining electrical devices and having a second non-planar surface,and the second bonding surface may include a polished, planarized andslightly etched deposited oxide layer on the second non-planar surface.

The first material may include a first wafer containing electricaldevices and having a first surface with irregular topology, and thefirst bonding surface may include a polished, planarized and slightlyetched deposited oxide layer on the first surface. The second materialmay include a second wafer containing electrical devices and having asecond surface with irregular topology, and the second bonding surfacemay include a polished, planarized and slightly etched deposited oxidelayer on the second surface.

The bonded device according to the invention may also include a firstmaterial having a first etched and activated bonding surface terminatedwith a first desired bonding species, and a second material having asecond etched and activated bonding surface terminated with a seconddesired bonding species bonded to the first bonding surface at roomtemperature.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof are readily obtained as the same become betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a flow chart of the method according to the invention;

FIG. 2 is a flow chart of an example of the method according to theinvention;

FIGS. 3A-3E are diagrams illustrating a first embodiment of a methodaccording to the invention;

FIGS. 4A-4E are diagrams illustrating bonding according to the inventionusing silicon oxide;

FIGS. 5A-5E are diagrams illustrating bonding according to the inventionusing silicon;

FIGS. 6A and 6B are graphs of room temperature bonding energy versusstorage time;

FIG. 7 is a diagram of a bonding fixture used in the invention; and

FIG. 8 is a fluorine concentration profile by SIMS (Secondary Ion MassSpectroscopy) near the bonding interface of deposited oxide coveredsilicon wafers that were very slight etched by diluted HF beforebonding.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 3A-3E, a first embodiment of the methodaccording to the invention will be described. Wafer 30, preferably aprocessed semiconductor device wafer and more preferably a processedsilicon device wafer, contains a device layer 31 with processed devices.Device layer 31 may contain a number of layers and include surfaceregions of wafer 30. The surface topography of layer 31 is typicallynonplanar. Layer 31 may also represent a processed integrated circuitcontaining any number of layers such as active devices, interconnection,insulation, etc.

The integrated circuit may be fully processed, or partially processedwhere the remaining processing is performed after the bonding process.The processing after the bonding may include full or partial substrateremoval or via formation between the bonded wafers for interconnection.

On layer 31 a bonding layer 32 is formed (step 1, FIG. 1). Bonding layer32 may be any solid state material or mixed materials which can bedeposited or formed at low temperatures and can be polished to asufficiently smooth surface. Layer 32 may be an insulator, such as SiO₂,silicon nitride, amorphous silicon formed using chemical vapordeposition (CVD) or plasma-enhanced CVD (PECVD), sputtering or byevaporation. Other materials such as polymers, semiconductors orsintered materials may also be used. Layer 32 should have thicknessgreater than the surface topography of layer 31.

The surface 33 of layer 32 is planarized and smoothed, as shown in step2 of FIG. 1 and in FIG. 3B. It is noted that the roughness/planarity ofsurface 33 is exaggerated in FIG. 3A for illustrative purposes. Thisstep may be accomplished using chemical-mechanical polishing. Surface 33is preferably polished to a roughness of about no more than about 3 nmand preferably no more than about 0.1 nm and be substantially planar.The surface roughness values are typically given as root-mean square(RMS) values. Also, the surface roughness may be given as mean valueswhich are nearly the same as the RMS values. After polishing surface 33is cleaned and dried to remove any residue from the polishing step.Polished surface 33 is preferably then rinsed with a solution.

The bonding surface may also be etched prior to polishing to improve theplanarity and/or surface roughness. The etching can be effective toremove high spots on the bonding surface by selective etching of thehigh spots using, for example, standard photolithographic techniques.For example, a layer of silicon nitride can be embedded within a silicondioxide bonding layer 32 that can serve as an etch stop when using asolution containing HF. The etch stop material may be used to improveuniformity, reproducibility, and manufacturability.

FIG. 3B illustrates layer 32 having upper surface 34 after thepolishing/planarization and cleaning steps. Surface 34 then undergoes anactivation process (step 3, FIG. 1). This activation process is anetching process and preferably a very slight etch (VSE) process. Theterm VSE means that the root-mean-square micro-roughness (RMS) of thevery slightly etched surface remains at approximately the unetchedvalue, typically <0.5 nm and preferably in the range of 0.1 nm to 3 nm.The optimum amount of material removed depends upon the material and themethod used for removal. Typical amounts removed vary from Angstroms toa few nanometers. It is also possible to remove more material. VSE alsoincludes the breaking of bonds on the treated surfaces and can occurwithout significant removal of material. The VSE is distinct from simplemodification of the surface by, for example, charging the surface withelectronic charge or damaging the surface layer. In a first example ofthe method according to the invention, the VSE process consists of a gasor mixed gas (such as oxygen, argon, nitrogen, CF₄, NH₃) plasma processat a specified power level for a specified time (FIG. 3C). The power andduration of the plasma process will vary depending upon the materialsused to obtain the desired bond energy. Examples are given below, but ingeneral, the power and duration will be determined empirically.

The plasma process may be conducted in different modes. Both reactiveion etch (RIE) and plasma modes may be used, as well as aninductively-coupled plasma mode (ICP). Sputtering may also be used. Dataand examples are given below in both the RIE and plasma modes.

The VSE process etches the surface very slightly via physical sputteringand/or chemical reaction and preferably is controlled to not degrade thesurface roughness of the bonding surfaces. The surface roughness mayeven be improved depending upon the VSE and materials etched. Almost anygas or gas mixture that will not etch surface 34 excessively can be usedfor the room temperature bonding method according to the invention.

The VSE serves to clean the surface and break bonds of the oxide on thewafer surface. The VSE process can thus enhance the surface activationsignificantly. A desired bonding species can be used to terminated onsurface 34 during the VSE by proper design of the VSE. Alternatively, apost-VSE treatment that activates and terminates the surface with adesired terminating species during the post-VSE process may be used.

The desired species further preferably forms a temporary bond to thesurface 34 atomic layer, effectively terminating the atomic layer, untila subsequent time that this surface can be brought together with asurface terminated by the same or another bonding species 36 as shown inFIG. 3D. Desired species on the surfaces will further preferably reactwith each other when they are in sufficiently close proximity allowingchemical bonding between surfaces 34 and 36 at low or room temperaturethat is enhanced by diffusion or dissociation and diffusion of thereacted desired species away from the bonding interface.

The post-VSE process preferably consists of immersion in a solutioncontaining a selected chemical to generate surface reactions that resultin terminating the bonding surface 34 with desired species. Theimmersion is preferably performed immediately after the VSE process. Thepost-VSE process may be performed in the same apparatus in which the VSEprocess is conducted. This is done most readily if both VSE and post-VSEprocesses are either dry, i.e, plasma, RIE, ICP, sputtering, etc, orwet, i.e., solution immersion. A desired species preferably consists ofa monolayer or a few monolayers of atoms or molecules.

The post-VSE process may also consist of a plasma, RIE, or other dryprocess whereby appropriate gas chemistries are introduced to result intermination of the surface with the desired species. The post-VSEprocess may also be a second VSE process. The termination process mayalso include a cleaning process where surface contaminants are removedwithout VSE. In this case, a post-cleaning process similar to thepost-VSE processes described above then results in a desired surfacetermination.

The post-VSE or post-cleaning process may or may not be needed toterminate surfaces with desired species if the activated surface bondsby the cleaning or VSE process are subsequently sufficiently weaklysurface reconstructed and can remain sufficiently clean before bondingsuch that subsequent bonding with a similar surface can form a chemicalbond.

The wafers are optionally rinsed then dried. Two wafers are bonded byaligning them (if necessary) and bringing them together to form abonding interface. As shown in FIG. 3D, a second wafer 35 has beenprocessed in the manner shown in FIG. 3C to prepare bonding surface 36.The two wafers are brought together by, for example, commerciallyavailable wafer bonding equipment (not shown) to initiate bondinginterface 37 (FIG. 3E).

A spontaneous bond then typically occurs at some location in the bondinginterface and propagates across the wafer. As the initial bond begins topropagate, a chemical reaction such as polymerization that results inchemical bonds takes place between species used to terminate surfaces 34and 36 when the surfaces are in sufficient proximity. The bonding energyis defined as the specific surface energy of one of the separatedsurfaces at the bonding interface that is partially debonded byinserting a wedge. The by-products of the reaction then diffuse awayfrom the bonding interface to the wafer edge or are absorbed by thewafers, typically in the surrounding materials. The by-products may alsobe converted to other by-products that diffuse away or are absorbed bythe wafers. The amount of covalent and/or ionic bonding may be increasedby removal of converted species resulting in further increase in bondstrength.

FIGS. 4A-4E show surface conditions and the bonding propagation to formcovalent bonds in a the case of a planar Si wafer covered with siliconoxide. On Si wafer 40 an SiO₂ layer 41 is formed, which has beenpolished and planarized. Surface 42 of layer 41 is subjected to the VSEprocess to produce an activated surface (FIG. 4A). On a second wafer 44a second SiO₂ layer 45 is formed, and surface 46 is subjected to a VSEprocess to activate surface 46 (FIG. 4B). Desired species are terminatedon surface 46 and are shown as lines 43 in FIG. 4C. Either or both of aVSE and post-VSE processes are used to properly terminate surface 46.While not shown, surface 42 may also be terminated using a post-VSEprocess. Wafer 44 is brought together with wafer 40 (FIG. 4D) and bonds46 begin to form. The bonding propagates and by-products are removed(indicated as arrows 47) and chemical bonds (such as covalent) areformed, as shown in FIG. 4E.

The bonding immediately after the RIE process may use a special bondingfixture allowing immediate in situ bonding of the etched wafers. Adiagram of the fixture is shown in FIG. 7. In plasma chamber 75 are twowafers to be bonded 70 disposed on RF electrodes 76 and 77. A plasma isformed in zone 79 by the application of RF power to the electrodes viamoveable vacuum RF power feedthrough 74 and by the introduction of anappropriate gas or gas mixture through gas feedthrough 73. Element 71 isa vacuum feedthrough for mechanical actuator (not shown) to retractretractable spacer 72. Chamber 75 is pumped down to a desired vacuumlevel via pumps (not shown) and chamber inlet 78. In the case where apost-VSE process or post cleaning process is also a dry process, asdiscussed above, the VSE and post-VSE or post-cleaning may be conductedin chamber 75.

After the plasma treatment to conduct the VSE process, the mechanicalspacers 72 are retracted by the mechanical actuator and the wafers 70are moved into contact with to begin the bonding process. The bondedwafers are then moved from the chamber into ambient or into anothervacuum chamber (not shown) and stored for a desired period to allow thebonding to propagate by a wafer handling system (not shown).

The materials of the bonding layers preferably have an open structure sothat the by-products of the polymerization reaction can be easilyremoved. The bonding species on the opposing bonding surfaces must beable to react at room temperature to form a strong or chemical bond. Thebond energy is sufficiently high to virtually eliminate slippage betweenwafers after subsequent heat treatments associated with a subsequentprocessing or operation when wafers have different thermal expansioncoefficients. Lack of slippage is manifest by a lack of wafer bowingupon inspection after the subsequent processing or operation.

In order to achieve the high bonding energies, it is preferable for atleast one of the wafers to be as thin as possible because a thin waferallows compliance to accommodate a lack of perfect surface planarizationand smoothness. Thinning to thickness of about 10 mils to 10 microns iseffective.

The bonded wafers are preferably stored at ambient or at low or roomtemperature after bonding to allow removal of species or convertedspecies for a specified period of time depending upon the materials andspecies used. Twenty four hours is usually preferable. The storage timeis dependent upon the type of plasma process used. Chemical bonds may beobtained more quickly, in a matter of minutes, when certain plasmaprocesses such as an Ar plasma are used. For example, 585 mJ/m² bondswere obtained in immediately after bonding and over 800 mJ/m² wereobserved after 8 hours for deposited oxides etched by an Ar plasmafollowed by NH₄OH dip.

Annealing the bonded wafers during bonding may increase the bondingstrength. The annealing temperature should be below 200EC and may betypically in the range of 75-100EC. Storing the bonded wafers undervacuum may facilitate the removal of residual gasses from the bondingsurfaces, but is not always necessary.

All of the processes above may be carried out at or near roomtemperature. The wafers are bonded with sufficient strength to allowsubsequent processing operations (lapping, polishing, substrate removal,chemical etching, lithography, masking, etc.). Bonding energies ofapproximately 500-2000 mJ/m² or more can be achieved (see FIG. 6A).

At this point (FIG. 3E) it is possible to remove a part or all of thesubstrate of wafer 35 by, for instance, lapping and etch back. The layerof devices of wafer 35 is thus transferred onto wafer 30. The devicesfrom the two layers may be interconnected. Additional device or circuitlayers may be bonded and interconnected to form a multilayer structure.Different types of wafers, devices or circuits may be bonded, as well asdifferent technologies (i.e. CMOS and bipolar or III-V HBT and Si CMOS).Other elements or materials such as thermal spreaders, surrogatesubstrates, antennas, wiring layers, a pre-formed multi-layerinterconnects, etc. may be bonded to produce different types of circuitsor systems, as desired.

In an example, shown in FIG. 2, PECVD SiO₂ is deposited on a Si wafercontaining devices. Surface 34, after the plasma (such as argon, oxygenor CF₄) treatment, is mainly terminated by Si—OH groups due to theavailability of moisture in the plasma system and in air. After theplasma treatment, the wafers are immediately immersed in solution suchas ammonium hydroxide (NH₄OH), NH₄F or HF for a period such as between10 and 120 seconds. After immersing the wafers in the NH₄OH solution,many Si—OH groups are replaced by Si—NH2 groups according to thefollowing substitution reaction:2Si—OH+2NH₄OH→2Si—NH₂+4HOH  (1)

Alternatively, many Si—F groups are terminating on the PECVD SiO₂surface after an NH₄F or HF immersion.

The hydrogen bonded Si—NH2: Si—OH groups or Si—NH2: Si—NH2 groups acrossthe bonding surfaces can polymerize at room temperature in formingSi—O—Si or Si—N—N—Si (or Si—N—Si) covalent bonds:Si—NH₂+Si—OH→Si—O—Si+NH₃  (2)Si—NH₂+Si—NH₂→Si—N—N—Si+2H₂  (3)

Alternatively, the HF or NH₄F dipped oxide surfaces are terminated bySi—F groups in addition to Si—OH groups. Since HF or NH₄F solutionetches silicon oxide strongly, their concentrations must be controlledto an adequately low level, and the immersion time must be sufficientlyshort. This is an example of a post-VSE process being a second VSEprocess. The covalent bonds across the bonding interface are formed dueto the polymerization reaction between hydrogen bonded Si—HF or Si—OHgroups:Si—HF+Si—HF→Si—F—F—Si+H₂  (4)Si—F+Si—OH→Si—O—Si+HF  (5)

FIG. 8 shows the fluorine concentration profile of bonded thermal oxidecovered silicon wafers that were dipped in 0.05% HF before roomtemperature bonding. A fluorine concentration peak is clearly seen atthe bonding interface. This provides evidence of the chemical processdescribed above where the desired species are located at the bondinginterface.

Since reaction (2) is reversible only at relatively high temperatures of˜500EC, the formed siloxane bonds should not be attacked by NH₃ at lowertemperatures. It is known that H₂ molecules are small and diffuse about50 times quicker than water molecules in oxide. The existence of adamaged layer near the surface of an adequate thickness i.e. a few nm,will facilitate the diffusion or dissolution of NH₃, and HF and hydrogenin reactions (2), (3), (4) and/or (5) in this layer and enhancement ofthe chemical bond. The three reactions result in a higher bonding energyof SiO₂/SiO₂ bonded pairs at room temperature after a period of storagetime to allow NH₃ or H₂ to diffuse away.

In the example of FIG. 2, the plasma treatment may create a damaged ordefective area in the oxide layer near the bonding surface. The zoneextends for a few monolayers. The damaged or defective area aids in theremoval of bonding by-products. Efficient removal of the bondingby-products improves the bonding strength since the by-products caninterfere with the bonding process by preventing high-strength bond fromforming.

Many different surfaces of materials may be smoothed and/or planarized,followed by a cleaning process, to prepare for bonding according to theinvention. These materials can be room temperature bonded by matingsurfaces with sufficient planarity, surface smoothness, and passivationthat includes cleaning, and/or VSE, activation and termination.Amorphous and sintered materials, non-planar integrated circuits, andsilicon wafers are examples of such materials. Single crystallinesemiconductor or insulating surfaces, such as SiO₂ or Si surfaces, canalso be provided with the desired surface roughness, planarity andcleanliness. Keeping the surfaces in high or ultra-high vacuumsimplifies obtaining surfaces sufficiently free of contamination andatomic reconstruction to achieve the strong bonding according to theinvention. Other semiconductor or insulator materials such as InP, GaAs,SiC, sapphire, etc., may also be used. Also, since PECVD SiO₂ may bedeposited on many types of materials at low temperatures, many differentcombinations of materials may be bonded according to the invention atroom temperature. Other materials may also be deposited as long asappropriate processes and chemical reactions are available for the VSE,surface activation, and termination.

For example, the method may also be used with silicon nitride as thebonding material. Silicon nitride may be bonded to silicon nitride, orto silicon dioxide and silicon. Silicon oxide may also be bonded tosilicon. Other types of dielectric materials may be bonded togetherincluding aluminum nitride and diamond-like carbon.

The method may be applied to planar wafers having no devices or circuitsand one wafer with devices and circuits. The planar wafer may be coatedwith a bonding layer, such as PECVD oxide or amorphous silicon, and thenprocessed as described above to bond the two wafers. The planar wafermay not need to be coated with a bonding layer if it has sufficientsmoothness and planarity and the proper bonding material.

As can be appreciated, the bonding process may be repeated with anynumber of wafers, materials or functional elements. For example, twodevice or IC wafers may be joined, followed by removing one of theexposed substrates to transfer a layer or more of devices, or just theactive regions of an IC.

The bonding according to the invention may be applied to joiningdifferent types of materials. For example, a silicon wafer can be bondedto another silicon wafer, or bond to an oxidized silicon wafer. The baresilicon wafer and the oxide covered wafer are immersed in HF, NH₄Fand/or NH₄OH and bonded after drying. The time for the immersion shouldbe less than about twenty minutes for the silicon wafer covered with thethin oxide since the NH₄OH solution etches silicon oxide. Since HF andNH₄F etches oxides strongly, very diluted solutions, preferably in0.01-0.2% range should be used for dipping of the silicon wafers.

After drying the silicon wafer and the oxide-covered wafer are bonded inambient at room temperature. Reactions (2), (3), (4) and/or (5) takeplace at the bonding interface between the two wafers. Theplasma-treated wafers may also be immersed in deionized water instead ofthe NH₄OH solution.

The silicon bonding may be conducted with a bare silicon wafer, i.e.having a native oxide or a silicon wafer having an oxide layer formed onits surface as described above. During the oxygen plasma treatment, thenative oxide which if formed on the bare silicon wafer is sputteretched, and the oxide layer formed on the silicon surface is etched. Thefinal surface is an activated (native or formed) oxide. When rinsed indeionized water, the activated oxide surface is mainly terminated withSi—OH groups. Since oxide growth in oxygen plasma has been found to haveless water than in normal native oxide layers, the water from theoriginal bonding bridge and generated by the following polymerizationreaction (6) can be absorbed into the plasma oxide readily.Si—OH+Si—OH→Si—O—Si+H₂O  (6)

FIGS. 5A-5E illustrate bonding two silicon wafers. Wafers 50 and 52 haverespective surfaces 51 and 53 with native oxides (not shown) subjectedto a VSE process (FIGS. 5A and 5B). Surface 53 is in FIG. 5C is shownterminated with a desired species 54. The two wafers are broughttogether and bonds 55 begin to form (FIG. 5D). The bonding propagatesand bonding by-products, in this case H₂ gas, are removed. Theby-products being removed are shown as arrows 56 in FIG. 5E.

In addition to removal of the water from the bonding interface bydissolving into the plasma activated oxide of the oxidized siliconwafer, the water can also diffuse through the thin oxide layer on thebare silicon wafer to react with silicon. As the silicon surfaceunderneath the oxide has a damaged or defective zone, extending for afew monolayers, the water molecules that diffuse through the oxide layerand reach the damaged or defective zone can be converted to hydrogen atroom temperature and be removed readily:Si+2H₂O→SiO₂+2H₂  (7)

The reverse reaction of (6) is thus avoided and the room temperaturebonding energy increases enormously due to the formation of covalentSi—O—Si bonds.

If a relatively thick (˜5 nm) oxide layer is formed, it will take a longperiod of time for the water molecules to diffuse through this thicklayer. On the other hand, if after the plasma treatment a thin oxidelayer is left or a too narrow defective zone is formed, water that canreach the silicon surface may not react sufficiently with the siliconand convert to hydrogen. In both cases the bonding energy enhancementwill be limited. The preferred oxygen plasma treatment thus leaves aminimum plasma oxide thickness (e.g., about 0.1-1.0 nm) and a reasonablythick defective zone (e.g., about 0.1-0.3 nm) on the silicon surface.

In a second embodiment, the VSE process uses wet chemicals. For example,an InP wafer having a deposited silicon oxide layer, as in the firstembodiment, and a device layer are bonded to a AlN substrate having adeposited oxide layer. After smoothing and planarizing the InP waferbonding surface and the AlN wafer bonding surface, both wafers arecleaned in an standard RCA cleaning solution. The wafers are veryslightly etched using a dilute HF aqueous solution with an HFconcentration preferably in the range of 0.01 to 0.2%. About a fewtenths of a nm is removed and the surface smoothness is not degraded asdetermined by AFM (atomic force microscope) measurements. Withoutdeionized water rinse, the wafers are spin dried and bonded in ambientair at room temperature. The resulting bonding energy has been measuredto reach ˜700 mJ/m² after storage in air. After annealing this bondedpair at 75° C. the bonding energy of 1500 mJ/m² was obtained. Thebonding energy has been measured to reach silicon bulk fracture energy(about 2500 mJ/m²) after annealing at 100° C. If the wafers are rinsedwith deionized water after the HF dip, the bonding energy at 100° C. isreduced to 200 mJ/m², that is about one tenth of that obtained withoutthe rinse. This illustrates the preference of F to OH as a terminatingspecies.

In a third embodiment the VSE process consists of 0.1% HF etchingfollowed by 5 min dip in 0.02% HN₄F solution of thermally oxidizedsilicon wafers at room temperature after a standard cleaning process.Without rinsing in deionized water, the wafers are bonded after spindrying at room temperature. The bonding energy of the bonded pairsreaches ˜1700 mJ/m² after 100° C. annealing. If the wafers are rinsed inde-ionized water after the HF etching before bonding, the bonding energyof bonded pairs is only 400 mJ/m², again illustrating the preference ofF to OH as a terminating species. Dilute NH₄F is used in the VSE processto etch silicon oxide covered wafers in a fourth embodiment. Theconcentration of the NH₄F should be below 0.02% to obtain the desiredbonding. The bonding energy of ˜600 mJ/m² can be achieved at roomtemperature after storage.

A fifth embodiment of the invention is used to bond Si surfaces having anative oxide of about 1 nm in thickness. In the fifth embodiment, aftercleaning the Si surface by a standard RCA1 cleaning process, a VSEprocess using 5 min etching in 70% HNO₃+diluted HF (preferably 0.01 to0.02%) is performed. Wafers are pulled out of the solution verticallywith a basically hydrophobic surface. Without rinsing in water, thewafers were bonded at room temperature in air. In this process covalentbonding occurs at room temperature with measured bonding energiestypically about 600 mJ/m². This bonding energy is significantlyincreased to 1300 mJ/m² after annealing at 75° C. and reaches thefracture energy of bulk silicon (about 2500 mJ/m²) at a temperature of100° C.

Instead of 70% HNO₃, diluted HNO₃ with water can be used in the solutionto achieve similar results. According to AMF measurements and highresolution transmission electron microscopy measurement results, thesilicon is etched in the dilute HNO₃ VSE process at a rate of 0.1-0.15nm/min and a new thick oxide 2.5-3.5 nm in thickness is formed.

As further embodiments, the VSE process may consist of a dry etch thathas chemical and/or physical components. For a bare Si surface, chemicaletching may result from SF₄/H₂ gas mixture while physical etching mayresult from Ar etch. For a silicon oxide surface, chemical etching mayuse CF₄ while physical etching may use oxygen or argon gas. It is alsopossible to use a thermally stable polymer material for the bondingmaterials and bond two polymer surfaces together. Examples arepolyimides or spin-on materials.

The mechanisms governing the increased bond energy at low or roomtemperature are similar. A very slight etching (VSE) of the bondingwafers by plasma to clean and activate the surfaces, and improve removalof by-products of interface polymerization to prevent the undesirablereverse reaction and rinse in appropriate solution to terminate thesurface with desired species to facilitate room temperature covalentbonding. The oxide covered wafer bonding case is similar except that adifferent surface termination is preferred. In bare silicon waferbonding, the highly reactive surface layers of oxide and silicon toallow water adsorption and conversion to hydrogen should be formed. Thehighly reactive layers can be a plasma thin oxide layer and a damagedsilicon surface layer. The oxide on the silicon wafer will also havesome damage. Not only O₂ plasma but also plasma of other gases (such asAr, CF₄) are adequate. Because during and after VSE the silicon surfaceis readily to react with moisture to form an oxide layer, and theunderlying damaged silicon layer is created by VSE. Since the VSE andby-products removal methods are rather general in nature, this approachcan be implemented by many means and apply to many materials.

EXAMPLE 1

In a first example, three inch <100>, 1-10 ohm-cm, boron doped siliconwafers were used. PECVD oxide was deposited on some of the siliconwafers. For comparison, thermal oxidized silicon wafers were alsostudied. The PECVD oxide thickness was 0.5 μm and 0.3 μm on the frontside and the back side of the wafers, respectively. Oxide is depositedon both sides of the wafer to minimize wafer bow during polishing andimprove planarization. A soft polish was performed to remove about 30 nmof the oxide and to smooth the front oxide surface originally having aroot mean square of the micro-roughness (RMS) of ˜0.56 nm to a final˜0.18 nm. A modified RCA1 solution was used to clean the wafer surfacesfollowed by spin-drying.

Two wafers were loaded into the plasma system, both wafers are placed onthe RF electrode and treated in plasma in RIE mode. For comparison, somewafers were treated in plasma mode in which the wafers were put on thegrounded electrode. An oxygen plasma was used with a nominal flow rateof 16 scc/m. The RF power was 20-400 W (typically 80 W) at 13.56 MHz andthe vacuum level was 100 mTorr. The oxide covered wafers were treated inplasma for times between 15 seconds to 5 minutes. The plasma treatedsilicon wafers were then dipped in an appropriate solution or rinse withde-ionized water followed by spin-drying and room temperature bonding inair. Some of the plasma treated wafers were also directly bonded in airwithout rinse or dipping.

The bonding energy was measured by inserting a wedge into the interfaceto measure the crack length according to the equation:

$\gamma = \frac{3\; t_{b}^{2}E_{1}t_{w\; 1}^{3}E_{2}t_{{tw}\; 2}^{3}}{16\;{L^{4}( {{E_{1}t_{w\; 1}^{3}} + {E_{2}t_{w\; 2}^{3}}} )}}$

E and tw are the Young's modulus and thickness for wafers one and twoand tb is the thickness of a wedge inserted between the two wafers thatresults in a wafer separation of length L from the edge of the wafers.

The room temperature bonding energy as a function of storage time ofbonded plasma treated oxide covered silicon wafers is shown in FIG. 6A.This figure shows measured room temperature bonding energy versusstorage time for 4 different cases as shown. The results can besummarized as follows: (1) for dipped and bonded RIE plasma treatedoxide wafers, the room temperature bonding energy increases with storagetime and reaches a stable value after ˜20 h in air or at low vacuum; (2)RIE mode results in higher bonding energies than plasma mode; (3) tooshort a plasma exposure time or too low a plasma power provides a smallor negligible increase in bond energy; (4) NH₄OH dip after plasmatreatment shows a much higher increase in bonding energy than waterrinse; (5) direct bonding in air after plasma treatment without dippingor rinse shows an almost constant bonding energy with time. The bondingenergy of the directly bonded wafer pairs immediately after roomtemperature bonding is slightly higher than the de-ionized water rinsedor NH₄OH dipped wafer pairs.

FIG. 6B shows room temperature bonding of Si and AlN wafers with PECVDoxide deposited layers. After about 100 h of storage time a bondingenergy of over 2000 mJ/m² were observed.

Comparing different bonding materials, the bonding energy as a functionof storage time of O₂ plasma treated thermally oxidized silicon waferpairs is similar to wafers with PECVD oxide, although the values of theroom temperature bonding energy are somewhat lower.

After ˜24 h storage in air at room temperature, the bonding energy ashigh as ˜1000 mJ/m² was reached in the RIE mode plasma treated and NH₄OHdipped PECVD oxide covered wafer pairs. Since the maximum bonding energyof a van der Waals bonded silicon oxide covered wafer pairs is about 200mJ/m², a large portion of the bonding energy is attributed to theformation of covalent bonds at the bonding interface at room temperatureaccording to the above equation.

EXAMPLES 2-3

The above process was applied to bond processed InP wafers (600 μmthick) to AlN wafers (380 μm thick), or to bond processed Si (380 μmthick) and InP 600 μm thick) wafers, as second and third examples. Theprocessed InP device wafers are covered with PECVD oxide and planarizedand smoothed by chemical-mechanical polishing CMP. A PECVD oxide layeris also deposited on the AlN wafers and is planarized and smoothed toimprove the RMS surface roughness. The processed Si and processed InPwafers are deposited with PECVD oxide and planarized and smoothed usingCMP. After VSE similar to the example 1 bonding at room temperature, thebonded wafers are left in ambient air at room temperature.

After 24 hours storage at room temperature, bonding energy of 1000 mJ/m²and 1100 mJ/m2 were achieved for the InP/Si and InP/AlN bonded pairs,respectively. For processed Si (380 μm thick)/oxide covered AN (280 μmthick) wafer pairs, the bonding energy at room temperature as high as2500 mJ/m² has been achieved. These room temperature bonded plasmatreated wafer pairs have sufficient bonding strength to sustainsubsequent substrate lapping and etching and other typical semiconductorfabrication processes before or after substrate removal.

The InP substrate in the room temperature bonded InP/AlN pairs waslapped with 1900# Al₂O₃ powder from initial 600 μm thick to ˜50 μm thickfollowed by etching in an HCl/H₃PO₄ solution to leave about a 2.0 μmthick InP device layer on the AN or Si wafer. The water and etchingsolution did not penetrate into the bonding interface.

Surfaces are sputter etched by energetic particles such as radicals,ions, photons and electrons in the plasma or RIE mode. For example, theO₂ plasma under conditions that bring about the desired VSE issputter-etching about 2 Å/min of PECVD oxide as measured by areflectance spectrometry. For thermal oxide the sputter etching rate isabout 0.5 Å/min. The thickness of oxide before and after plasmatreatment was measured by a reflectance spectrometry and averaged from98 measured points on each wafer. The etching by O₂ plasma has not onlycleaned the surface by oxidation and sputtering but also broken bonds ofthe oxide on the wafer surfaces.

However, the surface roughness of plasma treated oxide surfaces must notbe degraded by the etching process. AFM measurements show that comparedwith the initial surface roughness, the RMS of the O₂ plasma treatedoxide wafers was ˜2 Å and did not change noticeably. On the other hand,if the etching is not sufficiently strong, the bonding energyenhancement effect is also small. Keeping other conditions unchangedwhen the O₂ plasma treatment was performed with plasma mode rather thanRIE mode, the etching of oxide surfaces is negligible and the oxidethickness does not change. The final room temperature bonding energy isonly 385 mJ/m² compared to 1000 mJ/m² of RIE treated wafers (see FIG.6A).

Other gas plasma has shown a similar effect. CF₄/O₂ RIE was used toremove ˜4 nm of PECVD oxide from the wafer surfaces prior to bonding.The bonding energy of room temperature bonded PECVD oxide coveredsilicon wafers was also enhanced significantly in this manner andexceeds 1000 mJ/m² after sufficient storage time (see also FIG. 6A).

An argon plasma has also been used for the VSE with a nominal flow rateof 16 scc/m. The RF power was typically 60 W at 13.56 MHz and the vacuumlevel was 100 mTorr. The oxide covered silicon wafers were treated inplasma in RIE mode for times between 30 seconds to 2 minutes. The plasmatreated silicon wafers were then dipped in an NH₄OH solution followed byspin-drying and room temperature bonding in air. The bonding energyreached ˜800 mJ/m² at room temperature after only 8 h storage in air.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

The invention claimed is:
 1. A bonding method, comprising: forming aplanarized insulating material on a semiconductor wafer; etching theplanarized insulating material in a reaction space; terminating theplanarized insulating material with a nitrogen-containing species by atleast one of: a nitrogen-containing plasma during the etching, and anitrogen-containing solution after the etching; bringing a firstmaterial into direct contact with the terminated and planarizedinsulating material outside the reaction space; and forming a chemicalbond between the terminated and planarized insulating material and thefirst material.
 2. The method of claim 1, wherein the etching theplanarized insulating material is conducted in a vacuum chamber, thereaction space comprising the vacuum chamber.
 3. The method of claim 1,wherein the etching the planarized insulating material is conducted in aplasma chamber, the reaction space comprising the plasma chamber.
 4. Themethod of claim 1, wherein the etching and the terminating compriseexposing the planarized insulating material to the plasma etchingprocess in a plasma chamber, the reaction space comprising the plasmachamber.
 5. The method of claim 4, wherein the exposing the planarizedinsulating material to the plasma etching process comprises exposing theplanarized insulating material to a reactive ion etching process.
 6. Themethod of claim 4, wherein the exposing the planarized insulatingmaterial to the plasma etching process in the plasma chamber comprisesusing nitrogen gas.
 7. The method of claim 1, wherein the terminatingcomprises exposing the planarized insulating material to anitrogen-containing solution after the etching.
 8. The method of claim7, wherein the nitrogen-containing solution comprises an ammonia-basedsolution.
 9. The method of claim 1, wherein the first material comprisesa second planarized insulating material on a second semiconductor wafer.10. The method of claim 9, wherein the first material comprises siliconoxide.
 11. The method of claim 1, wherein the bringing into directcontact comprises bringing into direct contact the terminated planarizedinsulating material and a silicon material having a native oxide, thefirst material comprising the silicon material.
 12. The method of claim1, further comprising, after the terminating, rinsing the insulatingmaterial.
 13. The method of claim 12, wherein the rinsing comprisesimmersing the insulating material in deionized water.
 14. The method ofclaim 1, wherein the bringing into direct contact is performed at roomtemperature.
 15. The method of claim 1, wherein the bringing into directcontact is conducted in air.
 16. The method of claim 1, wherein theforming the planarized insulating material comprises forming siliconoxide on the semiconductor wafer.
 17. The method of claim 1, furthercomprising forming a bond between the planarized insulating material andthe first material with a strength of at least 500 mJ/m² withoutannealing at more than about 200° C.
 18. The method of claim 1, furthercomprising forming a bond between the planarized insulating material andthe first material with a strength of at least 2000 mJ/m² withoutannealing at more than about 200° C.
 19. The method of claim 1, furthercomprising annealing the planarized insulating material and the firstmaterial at a temperature of no more than about 200° C. after thebringing into direct contact.
 20. The method of claim 1, wherein theplanarized insulating material has a surface roughness between about 0.5and 1.5 nm after the etching and before the bringing into directcontact.
 21. A bonded structure comprising: a first semiconductormaterial comprising a planarized insulating material having a firstbonding surface; and a second material having a second bonding surface,wherein the first bonding surface is an etched surface terminated with anitrogen-containing species, and wherein the first and second bondingsurfaces are in direct contact with each other and bonded together witha chemical bond without any intervening adhesive.
 22. The structure ofclaim 21, further comprising at least one integrated circuit in thefirst semiconductor material.
 23. The structure of claim 21, wherein thesecond material comprises silicon.
 24. The structure of claim 21,wherein the chemical bond has a strength of at least 500 mJ/m².
 25. Thestructure of claim 21, wherein the chemical bond has a strength of atleast 2000 mJ/m².
 26. The structure of claim 21, wherein the planarizedinsulating material comprises silicon oxide.
 27. The structure of claim21, wherein the second bonding surface comprises an etched surface. 28.The structure of claim 27, wherein the etched surface of the secondbonding surface is terminated with a nitrogen-containing species. 29.The structure of claim 21, wherein the first bonding surface isterminated with a nitrogen-containing species by at least one of anitrogen-containing plasma during etching and a nitrogen-containingsolution after etching.
 30. The structure of claim 21, wherein thechemical bond is a covalent bond.
 31. A bonding method, comprising:forming a planarized insulating material on a semiconductor wafer;etching the planarized insulating material; terminating the planarizedinsulating material with a nitrogen-containing species by at least oneof: a nitrogen-containing plasma during the etching, and anitrogen-containing solution after the etching; bringing a firstmaterial into direct contact with the terminated and planarizedinsulating material; and forming a chemical bond between the terminatedand planarized insulating material and the first material.
 32. A bondingmethod, comprising: forming a planarized insulating material on aprocessed semiconductor wafer; exposing the planarized insulatingmaterial to a plasma process in a plasma chamber using anitrogen-containing gas; terminating the planarized insulating materialwith a species; bringing into direct contact the terminated andplanarized insulating material and a first material outside of saidplasma chamber; and forming a chemical bond between the terminated andplanarized insulating material and the first material.
 33. The bondingmethod of claim 32, wherein the species comprises nitrogen.